Control system



July 16, '1935. M. M. MoRAcK 21,008,512

CONTROL SYS'llEM Filed March 22, 1934 Y CRn-mm.

GRID VOLTAGE Inventor:

b q ttowned.

Patented July 1e, 193s PATENT 4ori-'ici'.

comor. SYSTEM Marvin M.l Morack, Schenectady, N. Y., assigner to GeneralElectric Company, a corporationof New York Application mmh zz, 1934i,serial No. 716,817 s om (ci. 11s-ago) This invention relates to controlsystems, more particularly to systems for controlling the opera.- tionof electroresponsive devices and the like, and

it has for an object the provision of a simple, re-

located electroresponsive device, such for example as a relay orcontactor, and it has for a further object the provision of means forcausing the device to operate in one manner at a certain value ofcontrol voltage, and to operate in an- T, other manner at asubstantially similar but slightly different value of control voltage;e; g. to cause a relay or contacter to pick up. and drop outatsubStantialIy the same `value of control voltage.

In one aspect this invention relates to illumination control systems andthe like. For example, in certain modern theatre dimming systems,

' the dimming apparatus comprising electric valves and saturablereactors are located behind or beneath the stage, and the dimmingcontrollers are remotely located on a panel or control desk in theauditorium from which the operator has a. view of the performance.Electrical connections are of course provided between the controllersand the dimming apparatus. These systems usually incorporate black-outcontactors for disconnecting a bank of lamps and its associatedsaturable reactor from the power supply when a rapid darkening orblack-out is desired. These black-out contactors are located near thedim- 35 ming apparatus. The provision of additional control connectionsbetween these black-out contactors and the control desk, which areundesir` able because of circuit complications and added 4characteristic, e. g. the varying voltage or current of the control deskconnections or of the dimming apparatus for actuating the contactor.'Ihis necessitates -a contactor which will pick up and drop out atsubstantially the same value of the controlling characteristic.Otherwise there wouldbe a. range of the controlling characteristic overwhich the contacter might be .either open or closed depending on theprevious 50 operation. Accordingly a further and more spe- I cilc objectof this invention is the provision of a remote control system in whichmeans are provided for causing a contactor to pick up and drop out atsubstantially the same value 'of an electrical characteristic f acontrolled device or exexpense, are averted by utilizing an electricalisting connections betwn a. controlling and a controlled device.

A further object of the invention is the provision of means for causinga large contactorv to pick up and drop out at substantially the samevalue of the controlling characteristics in systems in which the controlpower is very limited.

In carrying the invention into effect in one form thereof, anelectroresponsive device is connected in the output circuit of anelectric valve, which is capable, when fully conducting, of passingsufcient current to operate the electroresponsive device. A bias voltageis applied to the control grid of the valve and means are provided forapplying an alternating voltage to the output circuit and an alternatingvoltage of reverse phase is applied to the grid so that at a criticalvalue of bias voltage the valve is either fully conducting and theelectro-responsive device operated or the valve is non-conducting andthe device is not operated. A variable control voltage is utilized tovary the bias through a. range including this critical value.

In illustrating the invention in one form thereof, it is shown asembodied in a remote control theatre dimming .system for controlling theblack-out contactar.

For a better and more complete understanding of the invention referenceshould be had to the following specification and to the accompanyingdrawing in. which Fig. 1 is a simple diagrammatical illustration of anembodiment of the invention, Fig. 2 is a chart of operatingcharacteristics of a portion of the circuit, which facilitates anunderstanding. of the invention, and Fig. 3 is a modification. A

Referring now to the drawing, a. translating circuit, such for exampleas the circuit I0 across which is connected a plurality of load devicesi1- lustrated asa bank of lampsl I .is supplied with power from asuitable source of alternating voltage represented in the drawing by thetwo supply lines I2, I3'. The circuit IU is connected to the conductorI2 through the contacts lla of the black-out contactor I4 when thelatter is energized and the contacts Ila are bridged by the movablecontact member Hb. The other side of the lamp circuit Ill is connectedthrough the alternating current winding Ilot a saturable re- 'actor I8to the lower side I3 of the supply source. As shown, the saturablereactor is provided with `a. direct current saturating winding I1 towhich direct current of variable magnitude'is supplied by suitable meansillustrated as an electric valve Y n. simougntms valve may be o: any mimtype it is preferably of the three-electrode type vinto the envelope ofwhich a small quatity of an ionizable medium such for example as mercuryvapor is introduced after exhaust. The presence of this ionizable mediumwithin the tube serves to convert the usual electronic discharge into anarc stream thus constituting the tube an electrostatically or gridcontrolled arc-rectifier. The anode la. is connected through the directcrrent winding ll of the reactor to the upper supply line l2 by means ofa switching device 2l when the stationary contacts of the latter arebridged by the movable contacts.

The cathode i817 is connected to the lower supply line i3, as shown. Asthus connected, the valve i8 rectii'les alternate halves of thealternating voltage, e. g. it recties the positive half cycles and thisrectified voltage is supplied to the direct current Winding il of thereactor. Current iiow is sustained in the reactor winding during theinverse half cycles by means of a two electrode valve 2G, the anode 2da,of which is' connected to one terminal of the winding il and the cathode20h of which is connected to the opposite terminal through a currentlimiting resistance 22.

By varying the magnitude of the current owing in the control winding il,the reactance of the reactor and the reactive drop across thealternating current winding l5 thereof are varied in proportion. This ofcourse results `in varying the voltage drop across the lamp bank ll andhence the intensity of illumination of the lamp. In order to vary thecurrent in the direct cur- Y rent winding of the reactor, it isnecessary to vary the voltage -of the gridlac of the electric valve.

This is accomplished by means of a variable voltage device 23illustrated as an inductance capacitance potentiometer comprising acondenser 24 and a variable inductance 25 connected in parallelrelationship with each other and in series relationship with a condenser26 to the supply source l2, I3. 'I he grid ic of the electric valve I8is connected through a rectifying -device 27 and the alternating currentwinding of a scene fading saturable reactor 28 to an intermediate pointof the variable inductance 25.' Since the rectifying device 2l isincluded inthe grid connections, the voltage supplied to the grid ispartially rectified. The magnitude of this voltage may be varied asdesired by varying the inductance 25. Thus, by varying the inductance25, the grid voltage and anode current of the valve i8 are varied, whichas previously pointed out results in varying the intensity ofillumination of the lamp bank l I.

In certain installations a plurality of control devices similar to thedevice 23 are utilized for presetting a number of lighting scenes. Insuch installations, a "scene fader 30 controlling thev An illuminationcontrol system similar to the foregoing is fully disclosed and claimedin U. S. Patent 1,904,485, Livingston. The 'present invention mayhowever be applied to other forms of illumination control systems. Whena rapid darkening or black-out of the lamp ll is desired,

the black-out contactor M is utilized to disconneet the lamp Il and thesaturable reactor i6 from the supply source l2, I3. Inicertain im`stallations, the dimming apparatus, such for ex-l ample as the saturablereactors, electric valves and fading reactors illustrated in the drawingon the right hand side of the dotted line 3l is located in someconvenient place behind the stage or beneath it, Whereas the `controldevices 23 shown at the left-hand side of the dotted line si are usuallyarranged on a control panel or desk in the auditorium. With thearrangement shown in the drawing only a single conductor 32 is requiredbetween each control device 23 and its associated dimming apparatus. Ifindependent control of the black-out contactors lll were provided thiswould require an additional conductor from each black-out contactor tothe control panel. This is undesirable because of the added cost andcomplication of circuits. If dependent control of the black-outcontactcrs is provided it must not interfere with the normal control ofdimming apparatus by the control device 23. To meet these requirementsthe operation of the black-out contactors ll is controlled in accordancewith the magnitude of the voltage existing in the grid controlconnections 32 at a point physically near the dimming apparatus andremote from the control devices 23. In order to insure the necessarynon-interference with the regular dimming operation, the control of theblack-out contactors is so arranged that it does not become effectiveuntil near a limit of the range of the variable control voltage by meansof which the regular dimming operation is accomplished. Obviously, thecontactor le should pick up and drop out at substantially the same valueof control voltage or otherwise there would be a range of controlvoltages within vwhich the contactor Mlniight either be open or closeddepending upon the previous operation. If this were the case the controlwould be erratic, uncertain and unsatisfactory. It is, therefore, arequirement that the black-out contactors ll shall pick up and drop outat substantially the same` critical value of the control voltage.

Since 'direct current contactors are less noisy than alternating currentcontactors, the former are greatly preferred in theatres and similarinstallations. The contactor i4, therefore, is a directcurrentcontactor. Since the control volt-A age is alternating, means areprovided for translating the eiect of this alternating control voltageto the operating coil of the black-out contactor. As shown,directeur-rent is supplied to the operating coil of the black-outcontactor by means of an electric valve 33 which is of thethree-electrode type into the envelope of which a small quantity of anionizable medium such for example as mercury vapor or other inert gas isintroduced after exhaust. The presence of this ionizable medium servesto convert the usual electronic discharge into an arc stream therebyconstituting the valve an electrostatically or grid controlled arcrectifier. The valve is provided with an output circuit which includesthe anode 33a, a resistance 29 with which the operating coil of theblack-out contactor I4 is connected in parallel, and the cathode 33h.The grid 33e of valve 33 is connected to the control conductor 32through suitable rectifying means illustrated as a two electroderectifying valve 3Q. A

Alternating voltage is supplied to the output circuit of the valve 33 bymeans of a supply transformer 35, the primary winding 35a, of which isconnected to the supply source l2, i3, through the switching device A2landthe terminals of the secondary windings 35h of which are connectedous values of grid voltage which if exceeded in a positive directionwill initiate a discharge in the anode and cathode, In other words, ifat any instant during the positive half cycle of anode voltage the gridbecomes more positive than the critical grid voltage for that instant,the valve will fire and remain conducting through the remaining positivehalf cycle of anode voltage. It vwill thus be seen that by applying analternating voltage to the grid and varying its phase relationship withrespect to the anode voltage the instant at which the valve becomesconducting during eachv positive half cycle can be controlled and theaverage value of the current flowing in the output circuit can besimilarly controlled and varied as desired.

In the present case, however, variation of the output current isundesirable becausethe contactor I4 does not have substantiallyidentical pick-up and drop-out characteristics. This is averted byarranging the connections such that the valve is either fullyconducting'or non-conducting at a critical value of control voltage.Thus, when the valve is fully conducting the black-out contactor I4 isoperated, and when the valve is non-conducting the black-out contactorI4 is entirely deenergized. To accomplish this an alternating voltage ofsubstantially reverse phase with respect to the anode voltage -issupplied to the grid 33e of the valve by meansof the secondary biaswinding yo, which is energized from the primary winding 35a. The righthand terminal of the secondary winding 35o is connected both to the grid33C of the valve 33 and to the right hand anode of the rectfying. device34, Whilst its opposite terminal is connected through a filtercomprising a resistance and a parallel condenser 4| to the cathode ofthe valve 34. The rectifying valve 34 furnishes negative half waves ofvoltage which are partially smoothed out by the lter 40, 4| and thesecondary winding 35C supplies an alternating voltage to the grid. Ifthe instantaneous value of this alterhating voltage and the partiallyrectified voltage are added, the resultant voltage wave will have a.form similar to that represented by the curve 42. It will thus 'be seenthat this grid voltage represented by the curve 42 may be considered asconsisting of-an alternating voltage superimposed upon a. negative D. C.bias voltage. Thus, as

y p shown, the grid voltage 42 is always more negative than the criticalgrid voltage 31 during the positive half cycle of anode voltage.Consequently, there is no point during the positive half cycle 'of anodevoltage at which the valve 33 will fire and thus it remainsnon-conducting as long as this condition of grid voltage obtains.

However, as previously pointed out, the grid of valve 33 is connectedthrough the rectifying valve 34 and conductor 43 to the controlconductor 32. The connections are so chosen that positive half cyclesofthe alternating voltage of the control Conductor 32 are rectified bythe rectifying valve and this rectied voltage is smoothed by thefiltenshown as comprising the condenser 44 and parallel connectedresistance 45. When the instantaneous values of this rectified voltageare added to the instantaneous values of the grid voltage 42, the neteffect isto make the grid voltage more positive. As shown,diagrammatically, the net eilect is to displace the grid voltage curve42 upwardly. If the magnitude of the control voltage is sufficientlyhigh, the grid voltage curve 42 will be displaced to a positionindicated by the curve 4G. It will be noted that during the positivehalf cycle of anode voltage, the grid voltage represented by curve 45 isalways more positive than the `critical grid voltage and consequentlythe valve 33 conducts during the entire positive half cycle of anodevoltage. This means that the valve,33 is conducting the maximum` amountof current and the black-out contactor I4 is operated to its closedposition.

With the above understanding of the apparatus and its organization inthe completed system in mind, the operation of the system itself willreadily be understood from the following description:

It is assumed that the switching device 2| is operatedto itsclosedposition, thereby completing the output circuit of the electric valve I8AWhich supplies saturating current to the reactor o I6. If the lamp bankI I is to have a high intensity of illumination, the variable voltagedevice 23 is operated to increase the control voltage in the conductor32 to a high value and thus to cause a large current to ow in thesaturating winding of the 'reactor I6. This produces a decrease in thevoltage drop across the alternating current winding i5 of the reactorand increases the voltage across the lamp bank II. Since the voltagewinding I5 of the reactor and increases the voltage of the valve 33 asrepresented by the voltage A curve 45 is positive at the beginning ofthe positive half cycle of anode voltage, represented by curve 36.Consequently, the valve 33 conducts maximum current, the black-outcontactor I4 is operated to its closed position thereby completing theconnection of the lamp bank II and the saturable reactor I6 to thesource I2, I3.

If it is. now desired to dim the lamps II rapidly to a completeblack-out, the variable voltage device 23 is operated to decreasethevoltage in the control conductor 32. This decreases'the saturatingcurrent .of the reactor, thereby increasing its reactance and thevoltage drop across its alternating current winding which in turnresults in decreasing the voltage across the lamp bank II. Thedecreasing voltage in the control connection 32 decreases the positivevoltage supplied to the grid 330 of the valve 33. This has the effect ofdisplacing the grid voltage curve 46 in a downward direction. However.the current flowing in the output circuit of the valve 33 is notdecreased, but on the contrary the valve 33 remains fully conducting andthe black-out contactor I4 remains picked up. The explanation of this isthat although the grid voltage of the valve 33 is being decreased whichis represented by displacing the grid voltage curve 4S downward1y, thegrid voltage is still positive at the beginning of the half cycle ofanode voltage and consequently the valve conducts during the entire halfcycle. The electrical constants, of the circuit are so chosen that whenthe control voltage in the-conductor 32 has been reduced suiliciently toreduce the intensity of illumination ,of the lamp blank II toapproximately 10% of full intensity, the grid voltage of the valve -33is represented by the curve 48.

Itwill be observed that this curve crosses the critical -grid voltagecurve at the beginning of the positive half cycle of anode voltage.Consequently, the valve 33A is still fully conducting and CII theblack-out contactor is maintained in its closed position. I

However, if the control voltage ofthe connection 32 is now reduced avery slight amount the grid voltage of the valve 33 will also be reducedinproportion and the curve 48 willnow lie below the critical voltagecurve during the entire positive half cycle of anode voltage. Inothezrwords, at no time during the positive half cycle of anode voltage,does the grid become suiciently positive to initiate a discharge andconsequently the valve 33 is non-conducting and the black-out contactor'It is deenergized causing its movable contact member Mb to descend toits lower open position to disconnect the reactor I6 and the lamp bankII from the supply source I2, I3.

It will thus be seen that the operation of the valve 33 is extremelycritical at a predetermined value of the control voltage of theconductor 32, that is to say, a very minute change in the voltage atthis critical value varies the conductivity of the valve 33 from full-onto full-ofi.

Similary during the reverse operation, i. e. in-

' creasing the intensity of illumination of the lamp bank II, theoperation of the valve 33 is also very critical at this samepredetermined value of control voltage. As long as the control voltageis so low that the grid voltage of the valve 33 lies beneath the curve48 the valve 33 remains entirely non-conducting and the black-outcontactor remains unoperated. However, when the control voltage isincreased to the point at which the grid voltage of the valve 33 isrepresented by curve 48, the valve 33 becomes fully conducting and thecontacter I4 is operated to its closed position to connect the reactorI6 and the larnp bank I I to the supply source. I

The arrangement illustrated also improves the operation of theillumination system in the. following respect: The alternating currentwinding 28a of the scene' fading reactor 28 is connected to the sourceI2, I3 through the rectifying device 21 and consequently in the absenceof a provision of further means only half cycles of current would passthrough the reactive winding of the fader reactor. 'Ihe best operatingcharacteristics of the reactor are obtained, however,

, when a full wave alternating ,current iiows Cil through the reactivewinding. By connecting the rectifying valve 34 across the' conductor 32and the side I3 of the supply source in the manner illustrated theinverse half cycle of alternating voltage is rectied and consequentlyboth half cycles of alternating current flow through the reactor winding28a `therebyimproving its operation.

The modification illustrated in Fig. 3 is similar to the system of Fig.1, but ditt'ers therefrom primarily in the provision of means foreiecting operation of the controlled device in response to a D. C.control voltage. In certain electrical systems, s uch as rectiers anddirectcurrent transmission systems, there is a -need for a relay whichwill eiect certain control operations as a function of the directcurrent. particularly in'high voltage systems of this'char acter, toinsulate the controlling relays from the system voltage. A simplearrangement for accomplishing this object is illustrated in themodiication of Fig. 3. In this modication, it will be observed that theelectric valve 60 and its output circuit including the secondary windingSla of the supply transformer 6|,resistor 32 and contactor 63 areidentical with the electric valve .33 of Fig. 1 and its output circuit.The grid circuit It is desirable,

of the electric valve 60 is substantially identical type rectifyingdevice 64, such for example as a copper oxide rectier. A filter device,illustrated as comprising a resistance 65 and a condenser 66 connectedin parallel with each other, is provided for the purpose of smoothingthe rectified bias voltage.

The direct current circuit, the current of which is to be utilizedforcontrolling the operation oi the relay 63, is represented by the plusand minus lines 61. As shown, the grid circuit of the electric valve 60is connected to this direct current circuit, but insulated therefrom, bymeans of a saturable core type reactance device B8. The alternatingcurrent winding 68a of the saturable core reactor is supplied withalternating voltage from the secondary winding GIC of the supplytransformer. A` suitable rectifying device, illustrated as a surfacecontact rectifying bridge 10 is included in circuit between thesecondary winding `6Ic and the alternating current winding 68a of Inoperation, the impedance of the alternatin current winding 68a of thereactor varies inversely with the direct current flowing in thesaturating windings 68h and 68e. For example, as the current in thesaturating winding increases the impedance of the alternating currentwinding decreases and the rectified voltage appearing across theresistance. 'II increases. The increase in the voltage across theresistance 1I has the same eiect as an increase in voltagev across theresistance has in the system of Fig. 1. In other words, a variation inthe current flowing in the D. C. circuit 61 has the same controllingeffect on the electric valve 60 and its associated relay 63 that avariation in the voltage of thecontrol connection 32 in the system ofFig. 1 has on the electric valve 33 and its associated contactor I 4.For currents in the saturating winding of the reactor which are lowerthan the desired trip value of the relay, the actual grid voltage of theelectric valve 60 is more negative than the-critical grid voltage. Asthe current which iiows in the saturating winding increases, thepositive bias across the resistor 'II increases until the actual gridvoltage curve Aintersect's the critical grid voltage, thereby causingthe valve 60 to become ionized at the beginning of each positive halfcycle of anode voltage. The valve 6B, therefore, becomes fullyconducting and energizes therelay 63, causing it to close its contacts.SimilarlyLwhen the current owing in the saturating winding becomes lessthan the vcritical value, the grid voltage of the valve 63 is 2,008,512cuit between the'recufying bridge and the man resistance ll of the gridcircuit. This variable resistance 'N serves to vary or adjustthe settingof the relay, i. e. the current in the saturating winding of the reactorat which the relay is energized.

Although in accordance with the provisions of the patent statutes, thisinvention is embodied in concrete form, it will be understood that theapparatus and connections shown are merely illustratve and the inventionis not limited thereto since alterations and modiilcations will readilysuggest themselves to persons skilled in the art Without departing fromthe true spirit of the invention or from the scope of the annexedclaims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A control system comprising an electroresponsive device, an electricvalve having an ionizable medium, a control grid and an output circuitconnected to said device capable of passing suicient current to operatesaid device when said valve is fully conducting, means for supplying anegative bias voltage to said grid, means for.sup plying an alternatingvoltage to said output cir- -cuit and an alternating voltage of reversephase to said grid, and means for supplying a positive bias voltage tosaid grid to reduce said negative bias voltage so that said valve isrendered fully conducting and said electroresponsive device is operatedfor certain values of said bias andsaid valve is rendered non-conductingand said` electroresponsive device is not operated for all other values.

2. A control system comprising an electric .l valve containing anionizable medium, and having an output circuit and a control grid, anelectroresponsive device connected to said output circuit and capable ofactuation when said valve is fully conducting, a half wave rectierconnected in the grid circuit for supplying a negative bias voltagethereto, means for supplying an alternating voltage to said outputcircuit and an alternating voltage of reverse phase to said grid, andmeans for supplying a positive voltage to said grid to oppose saidnegative bias voltage so that said valve is fully conducting and saidelectroresponsive -device actuated for certain values of so that saidvalve is fully conducting and saidelectroresponsive deviceis operatedfor certain values of said bias and said valve is non-conducting andsaid device is not actuated for all other values, and means forsupplying a variable positive voltage to said grid to decrease saidnegative bias tothe value at which said valve becomes fully conducting.-j

4. A control system comprising van electric valve containing anionizable medium, a cathode, an anode and a control grid, anelectroresponsive device connected in the anode circuit of said valve, asource oi' alternating voltage, connectlons from said source to saidanode circuit for supplying an alternating voltage thereto,

connections from said source to said grid including a rectifying' devicefor supplying a negative bias and a superimposed alternating voltage ofreverse phase with respect to the anode voltage so that said valve isconducting and said l electroresponsive device is actuated for certainvalues of said bias voltage less negative than a predetermined criticalvalue and said valve is non-conducting and said device not actuated forall other values of said bias, and means for supplying a variablepositive"'voltage to said grid to vary said bias voltage through all ofsaid values. 5. A control system comprising an electric valve containingcathode, anode. control grid and an-ionizable medium, anelectroresponsivedevice connected ln the anode circuit of said valve, asource of alternating voltage, connections from said'source to the anodecircuit of said valve for applying an alternating voltage thereto,connections from said source to said grid including a half waverectifying device for supplying to said grid a negative bias and asuperimposed alternating voltage of reverse phase with respect to I saidanode voltage whereby said valve is non-conducting for certain values`of said bias and fully conducting for all other' values,- and meansforl supplying an alternating current of variable effective value-tosaid rectifying device whereby a variable positive voltage is suppliedto said grid for decreasing said bias to cause said valve to becomeconducting.

6.v A control system comprising an electric valve containing cathode,anode, control grid and an lonizable medium, an electroresponsive deviceconnected in the anode circuit of said valve, a source of Aalternatingvoltage, connections from said source to said anode for supplying 'analternating voltage thereto, a rectifying device having a plurality ofanodes, connections including one of the anodes'of said rectifyingdevice from said source to said grid for supplying to said grid anegative biasv and a superimposed alternating voltage of reverse phasewith respect to the anode voltage of said valve so that said valve isfully conducting for certain values of said bias and non-conducting forall other values, and connections from another anode of said rectifyingdevice to a source of alternating voltage of variable effective valuefor decreasing said negative bias to render said valve conducting.

7. A relay control system comprising a source of alternating voltage, anelectric valve containing'cathode, anode, control grid and an ionizablemedium, a source of alternating voltage, a relay having its operatingcoil connected in the anode circuit of said valve, a rectifying devicein the ternating voltage thereto, connections from said source to saidanode circuit for supplying an al- 'ternating voltage thereto,connections from said source to said grid and said rectifying device forapplying vto said grid a negative bias voltage and a super-imposedalternating voltage of reverse phase with respect to said an'ode voltageso that said valve conducts suicient current to actuate said relay forcertain values of said biasA and is non-conducting for all other valuesof said bias, a source of variable control voltage and connections fromsaidcontrol voltage source to said grid for supplying a variablepositive voltage to said grid ,to vary saidbias through all of saidvalues so that said relay is actuated for certain values of said controlvoltage and deenergized for all other values.

8. A control system comprising a source'of vato be controlled by' saidvoltage, connections from said source to said device, an electric valvecontaining an ionizable medium and having a control grid and an outputcircuit, an electroresponsive'device connected in said output circuitand operable to exert control over said controlled device when saidvalve is fully conducting, means for supplying a negative bias to saidgrid, means for supplying an alternating voltage to said out- 10 putcircuit and an alternating voltage of reverse phase to said grid so thatsaid valve is fully conducting for certain values of said bias andnonconducting for all other values, and connections from said grid tosaid control voltage connections for supplying a variable positivevoltage to said grid to vary said bias to provide actuation of saidelectroresponsive device for certain values of said control voltage anddeactuation for all other values.

MARVIN M. MORACK.

CERTIFICATE 0F CORRECTION.

'9mm No'. 2,008,512. my 16, 1935.

MARviN M. MoRAcK.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiringcorrection as' follows: Page 3,second column, line 33, for the words "winding 15 of the reactor andincreases the" read in the control conductor 32 is high, the grid;andlne' 68, for "blank" read bank; page 5, line 11, first column,

for "illustratve" read illustrative; and vsecond column, line 57, claim7, strike out the syllable and words "ternating voltage thereto" andinsert instead grid circuit of said valve; and that the said LettersPatent should be read with these corrections therein that the same mayconform to the re'cord of the case in the .Patent Office.

g Signed and sealed this 27th day-of August, A. D. 1935.

Les] ie Frazer v1(3gal) Acting Commissioner of Patents.

CERTIFICATE 0F CORRECTIGN.

'Pmm No; 2,008,512. July 16, 1935.

MARVN M. MoRACK.

l lt is hereby certified that error appears in the printed specificationof the above numbered patent requiring'correction as' follows: Page 3,second column, line 33, for the words "winding 15 of the reactor andincreases the" read in the control conductor 32 is high, the grid;andiine' 68, for "blank" read bank; page 5, line 1l, first column,

for "illustratve" read illustrative; and second column, line 57, claim7, strike out the syllable and words "ternating voltage thereto" andinsert instead grid circuit of said valve; and that the said LettersPatent should be read with these corrections therein that the same mayconform to the record of the case in the IPatent Office.

i Signed and sealed this 27th day-of August, A. D. 1935.A

Leslie FrazerA v(grml) Acting Commissioner of Patents.

